Metal oxide semiconductor field-effect transistor (MOSFET) devices can be used as switches for coupling electronic signals. Generally, they demonstrate little or no offset voltages that can be found in bipolar and other solid state switches. Typically, MOSFET switch on resistance (Ron) is very low and the off resistance (Roff) is very high. In modern devices, Ron can be on the order of an ohm or less and Roff can be many megohms. In MOSFET switches, Ron can be a function of the gate to source voltage, Vgs, of the device. Since Ron is a function of Vgs, Ron can be reasonably constant if Vgs is constant, other conditions being equal. When such switch devices are used for audio signals, the fidelity of the transferred signal across the switch can be negatively affected if Ron changes with the input signal voltage level. One measure of the audio fidelity of a switch may be the total harmonic distortion (THD) that is introduced by the switch.
Keeping MOSFET Ron constant despite input signal voltage variations is an object of Pollitt U.S. Pat. No. 4,093,874, entitled, “Constant Impedance MOSFET Switch” (herein, “the '874 patent”). The '874 patent discusses keeping Ron constant by keeping Vgs constant (at a set temperature) regardless of changes of the input signal voltage. The '874 patent, however, uses the logic signal voltage value, which turns the switch on/off, to determine the Vgs voltage level. However, the logic signal voltage value can change as the power supply voltage, which produces the logic signal voltage, changes with a load. Such changes in Vgs can change Ron and, thus, limit the useful dynamic range of the '874 patent. In addition, in applications with large negative signal swings, the reliability of a MOSFET can be compromised due to degradation of the MOSFET's gate oxide.